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Abstract: While closed-circuit hydrostatic transmissions (HTs) achieve remarkable efficiencies (≤98%) in their main pump-motor units, auxiliary charge pump systems remain a significant source of avoidable energy loss. This review synthesizes current research to demonstrate that conventional fixed-operation charge pumps-operating continuously at ~25 bar pressure and 10-20% of main pump flow-waste substantial energy due to misalignment with actual system demands. Charge pumps perform critical functions: compensating for internal leakage, providing filtration/cooling, supplying servo-control pressure, preventing cavitation, and ensuring lubrication. However, their static operation causes continuous throttling losses and heat generation. Recent advances in variable-displacement pumps and adaptive control strategies (e.g., pressure compensation, servo-proportional control) enable dynamic adjustment of charge pressure and flow, unlocking 20-45% energy savings without compromising functionality. We identify charge system optimization as the next frontier for HT sustainability and prioritize research directions: robust adaptive algorithms, digital hydraulic integration, and standardized validation frameworks.

Keywords: Hydrostatic transmission, charge system optimization, adaptive control, energy efficiency

1. Introduction - The critical imperative for charge system optimization in hydrostatic transmissions

1.1 The global energy efficiency challenge

The escalating global demand for energy sustainability has intensified scrutiny of industrial systems, where fluid power technology remains indispensable for high-power applications. With industrial hydraulics consuming approximately 2% of global electricity [1], even marginal efficiency gains yield substantial environmental and economic returns. Hydrostatic transmissions (HTs) -particularly closed-circuit configurations - stand at the forefront of this effort, offering unparalleled power density, precise controllability, and bidirectional operation in applications ranging from construction machinery to wind turbines [2, 3]. Recent advances in axial-piston pump-motor units have pushed their global efficiency (ηg) to remarkable levels (≤98%), nearing thermodynamic limits imposed by fluid viscosity and mechanical friction [1, 7, 11]. Yet paradoxically, this pursuit of component-level perfection has obscured a critical subsystem-level inefficiency: the auxiliary charge pump system, whose static operation represents a persistent source of avoidable energy waste.

1.2 Hydrostatic transmissions: Architecture and efficiency frontiers

Closed-circuit HTs feature a sealed hydraulic loop where a bidirectional pump (PHS) directly drives a hydraulic motor (MHS), eliminating directional valves and reservoir intermediation (Fig. 1). This architecture enables exceptional power transfer efficiency through:

. Direct power coupling: No throttling losses from control valves [3, 19] . Advanced tribology: Diamond-like carbon (DLC) coatings reducing friction losses [7] . Precision fluid dynamics:...

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